In general, spark plugs include an insulative sleeve having a central axial bore through which a center electrode extends. The insulating sleeve is positioned within, and secured to, a metal shell that serves as a mounting platform and interface to an internal combustion engine. The metal sleeve also supports a ground electrode that is positioned in a particular spaced relationship relative to the center electrode so as to generate a spark gap. The insulating sleeve includes a shaped tip portion that resides in a recessed end portion of the metal shell. The shaped tip portion is configured to protect the electrode from engine heat and products of combustion. The spark plug is typically mounted to an engine cylinder head and selectively activated to ignite a fuel/air mixture in an associated engine cylinder.
Over time, products of combustion or combustion deposits build up around the center electrode and insulative sleeve, particularly the shaped tip portion. This build up of combustion product inhibits spark formation across the spark gap. A significant build up of combustion products may foul the spark plug and result in ignition failure, i.e., the combustion products completely block the spark from forming between the center and ground electrodes due to an electrical short circuit formed from the combustion products. Combustion deposit build up is particularly problematic during cold starts. During cold starts, complete combustion of the air/fuel mixture is seldom achieved which results in an increased generation of electrically conductive combustion deposits. As a result of continuous cold starts, electrically conductive combustion deposits build up, resulting in an electrical short circuit between the center electrode and the electrically grounded portion of the spark plug.
Previous, attempts to address combustion deposit build up issues have included silicone oil coatings and particulate vanadium oxide deposition on the insulating sleeve. These coatings have failed to adequately address the issue—suffering from inadequate performance at elevated temperature, inadequate endurance, or insufficient reduction of combustion deposit build up.
Accordingly, there is a need for a spark plug which has a decreased susceptibility to electrically conductive combustion deposit build up in the insulative sleeve.